Ideally you would have a series of referral records from the customer (and each record would include a referral date.) If you add a column to capture their portal setup date, you could color your viz (whether line or bars or whatever) to have one color before-portal, and another after-portal. Each customer would have his own portal date, so if you have 15 separate lines for 15 customers on the viz, each would have the break in color at his respective portal setup date.